Methods and apparatus for zone melting



Feb. 18, 1969 w. F. TUCKER 3,428,436

METHODS AND APPARATUS FOR ZONE MELTING Filed Dec. 16, 1963 Sheet of 2 2a46 30 2s 52 32 49 36 9| s4, as 42 50 40 48 I 54 59 56 H 60 6' /sa H.5Vdcl 76 /ooo aza- L IO 5 2 I8 I INVENTOR. W'LLIAM F. TUCKER Feb. 18, 1969w. F. TUCKER 3,423,436

METHODS AND APPARATUS FOR ZONE MELTING Filed Dec. 16, 1963 Sheet 3 SOLID66 LIQUID FIG. 2

SOLID LIQUID 92 SOLID I FIG. 5'

INVENTOR.

WILLIAM F. TUCKE R United States Patent 3,428,436 METHODS AND APPARATUSFOR ZONE MELTING William F. Tucker, Creve Coeur, Mo., assignor toMonsanto Company, St. Louis, Mo., a corporation of Delaware Filed Dec.16, 1963, Ser. No. 330,671

US. Cl. 23-301 17 Claims Int. Cl. B01 17/16 This invention relates tomethods and apparatus for Zone melting semiconductor materials and moreparticularly to methods and apparatus for zone melting a verticallypositioned rod of semiconductor material such that the rod segmentsolidified from the molten zone is of relatively constant diameter.

Semiconductor materials are zone melted for a number of purposes. Forexample, zone melting operations are conducted to effect purification ofsemiconductor materials, to effect uniform distribution of carrierimpurities in semiconductor materials, and to effect the transformationof polycrystalline semiconductor materials into a rod of semiconductormaterial formed from a single crystal. In other instances the zonemelting operation is conducted primarily to effect a change in diameterof a rod of Semiconductor material or to remove irregularities in thediameter of a rod material. Instances where it is desirable to effect achange in the diameter of a rod of semiconductor material include theproduction of slim rods for use in a decomposer and, in many instances,the production of seeds for use in the manufacture of single crystalmaterial.

In all operations of the above type it is normally desirable that therod of semiconductor material resulting from the zone melting operationbe predominantly of an uniform diameter. For example, in the manufactureof slim rods for use in a decomposer if the slim rods are irregular indiameter, the irregularities are amplified during the deposition ofpolycrystalline material upon the slim rods so that the resultingpolycrystalline rod is highly nonuniform in diameter. This results indifficulties if one attempts to employ the polycrystalline rod in a zonemelting operation and, because of its nonuniform diameter, one cannoteasily subdivide it into chunks of substantially equal weight for use ascharges for a crystal puller. As another example, if one is producingrods of extremely small diameter for use in the manufacture of seedcrystals, nonuniform diameter seeds result in alignment difficultieswhen the seeds are used for the manufacture of pulled crystals and theseeds are also less satisfactory for use in the preparation of singlecrystal rods by zone refining techniques.

The need for means to produce a zone melted rod of uniform diameter hasbeen previously recognized and substantially all zone melting apparatusis provided with either a manual or automatic means for rod diametercontrol. For the production of small diameter rods diameter control ismore difficult and automatic diameter control means are normallyembodied into apparatus for producing such rods, the most usual type ofautomatic diameter control means being as disclosed in US. Patent2,992,311 issued to Wolfgang Keller. In accordance with this US. patent,photoelectric means are provided for measuring the diameter of a rod ofzone refined material at the point where resolidification of the moltenzone is effected. Such a system is satisfactory in the production ofrods down to about millimeters in diameter since with rods of thisdiameter or larger it is possible to maintain diameter fluctuationsbelow about percent; however, in the production of rods of diametersless than about 4 or 5 millimeters, apparatus in accordance with theKeller patent is not entirely satisfactory. For example, in the "iceproduction of slim rods of 3.7 millimeters diameter it is difficult tomaintain diameter fluctuations below about 30 percent using apparatus asdisclosed in the Keller patent. Uniform semiconductor rods of less than4 millimeters diameter are advantageous for use in the production ofseeds and in the production of slim rods for decomposers and a methodand apparatus for producing such rods represents a material advance inthe art.

It is an object of this invention to provide improved methods andapparatus for producing zone melted rods of semiconducted material ofrelatively uniform diameters.

It is another object of this invention to provide methods and apparatusfor producing very small diameter semiconductor rods which vary lessthan about 10 percent from a mean diameter.

The above as Well as other objects of the invention are accomplished byapparatus including means to support at either end of a rod ofsemiconductor material, means to form a molten zone in the rodintermediate to its ends, and means to control the relative movement ofthe two ends in response to variations in the displacement of theinterface of resolidification, which forms one longitudinal boundry ofthe molten zone, from the mean plane of the heating means or anyarbitrarily selected reference plane extending transversely through therod of semiconductor material. It has been found that fluctuations inrod diameter which occur with apparatus embodying a conventiona1automatic diameter control system are partially or largely the result ofchanges in the degree of heater coupling with perceptible diameterchanges. It has further been found that the interface ofresolidification makes a discernible move relative to the heater priorto the time that a readily discernible change in diameter of the rodbeing solidified from the molten zone has occurred and that if thecontrol system is responsible partially or totally to this movement ofthe interface of resolidification, rather than solely to changes in roddiameter, that changes in rod diameter can be maintained at a very lowlevel.

Several specific embodiments of the invention will now be described withreference to the accompanying drawings in which FIGURE 1 is a schematicdrawing of a slim rod pulling apparatus designed to produce a rod ofsmall diameter from a larger diameter rod as a result of a zone meltingoperation and which embodies rod diameter control means in accordancewith this invention.

FIGURE 2 is an enlarged view of the molten zone with adjacent portionsof each of the solid ends of a rod of semiconductor material being zonemelted in the apparatus of FIGURE 1.

FIGURE 3 is an enlarged view of the molten zone and adjacent portions ofa rod of semiconductor material showing an alternative arrangement inwhich not only the position of the interface of resolidification isdetected but in which the diameter of the rod at the interface ofresolidification is also detected using a separate' photoelectric cell.

FIGURE 4 is an enlarged view of a. molten zone as in FIGURE 3 wherein asingle photoelectric cell is used to detect both the position of theinterface of resolidification and the diameter of the rod at theinterface of resolidification.

With particular reference to FIGURE 1 of the drawings there isillustrated a lower rod holding means 10 designed to secure the lowerend of a rod of semiconductor material 12 in a vertical position. Therod holder 10 is supported upon a threaded vertically disposed shaft 14which engages an internally threaded collar 16 carrying a horizontallydisposed gear 18, the collar 16 being rotatably supported upon a supportframe member 20 board to receive the vertical shaft 14. Horizontallydisposed gear 18 is in operative engagement with a second gear 22 whichis driven through a conventional reduction gear train, not illustrated,by a suitable electric motor 24. Shaft 14 is free to move vertically butis keyed against rotation so that by rotation of gear 18, the shaft 14and rod holder can be moved vertically as desired.

The reference numeral 26 generally indicates a second rod holding meanswhich in this instance is shown to comprise a pair of grooved rollers 28and 30. The roller 28 is mounted upon a shaft 32 which is journaledthrough a pair of support arms, one of which is shown at 34, on eitherside of the roller 28. Roller 30 is similarly mounted upon a shaft 36which is journaled at either end through a pair of support arms one ofwhich is shown at 38. Support arms 34 and 38 are respectively carried bya pair of shafts 40 and 42 which are journaled through a support member44. The support arms 34 and 38 are rotatable about shafts 40 and 42respectively but relative movement between the arms and shafts 40 and 42is preferably not friction free for reasons which will subsequently bemade apparent.

Secured to one end of roller 28 is a gear 46 which meshes with a gear 48secured to shaft 40. Similarly, secured to one end of roller 30 is agear 49 which meshes with a gear 50 secured to shaft 42 and which inturn meshes with gear 48 so that gear 48 drives both rollers 28 and 30but in opposite directions. A suitable electric motor schematicallyillustrated at 52 is provided for driving shaft 40, to which gear 48 issecured, through a suitable conventional gear reduction train, notillustrated.

The reference numeral 54 designates a rotary support platform upon whichthe rod holding means 26 is positioned. While it is not necessary thatplatform 54 rotate it has been found and is well known in the art thatrotation of one or both rod holding means securing the ends of a rod ofsemiconductor material being zone refined is normally desirable andgives improved results. A small pulley 56 driven by an electric motorschematically illustrated at 58 is provided for rotating platform 54 bymeans of a belt 59 running in a suitable groove in the periphery of theplatform. To reduce vibration and assist in maintaining the upper solidportion of rod 12 in proper alignment, a pair of freely rotatable idlerrollers 60 and 61 are disposed below platform 54 and are yieldablybiased toward each other to form a nip through which rod 12 passes.

The reference numeral 62 designates a convention RF (high frequency)heater coil operatively disposed around the rod of semiconductormaterial 12. The heater coil 62 is electrically connected to a suitablesource of high frequency current such as a conventional high frequencygenerator schematically illustrated at 64 so that by passing a highfrequency current through the coil 62 a molten zone 66 can be created inthe upstanding rod of semiconductor material 12. As will subsequently bemade clear, the molten zone 66 is bounded at its upper extremity by atransverse interface of resolidification 68 and at its lower extremityby a transverse interface of melting 70.

The reference numeral 72 generally indicates means for sensingvariations in the distance between the transverse interface ofresolidification 68 and an arbitrarily selected reference plane passingtransversely through the rod of semiconductor material and movingrelative to the longitudinal axis of rod 12 at a rate equal to the meanrate of movement of molten zone 66. For purposes of illustration areference plane has arbitrarily been selected to correspond to the uppersurface of the heater coil 62 and is shown by dotted lines in FIG- U-RE1 of the drawings and indicated by the reference letter a, and thedistance between the selected reference plane and the interface ofresolidification is indicated by the reference letter d. In actualpractice the desired result can readily be obtained by mounting thesensing means 72 on the bracket or platform which supports heating coil62 so that the sensing means is in fixed positional relationship to theheater coil.

The distance measuring means 72 is illustrated as comprising aphotoelectric cell 74, which in this instance can suitably be a variableresistance cadmium selenide cell, and a lens system schematicallyillustrated at '76. As best shown in FIGURE 2 of the drawings, lens 76produces an image upon the surface of photoelectric cell 74 of a portionof the surface of rod 12 including adjacent surface areas of the solidportion of rod 12 above interface 68 and of the molten zone 66. Withthis arrangement vertical movement of the interface of resolidification68 relative to plane a results in a change in the radiation received bythe surface of cell 74 due to a difference in the intensity of radiationbeing emitted from the solid and molten areas of rod 12.

The photoelectric cell 74 is connected through suitable electrical leadsto a magnetic amplifier, indicated by the reference numeral 78, which inturn is connected through suitable leads 79 and to a source ofalternating current electricity. Suitable magnetic amplifiers for use inaccordance with this invention are commercially available and may beobtained from a number of different manufacturers. The photoelectriccell 74 is connected to the magnetic amplifier 78 such that a decreasein the radiation being received upon the surface of the photoelectriccell through lens 76 results in an increase in the resistance of thephotoelectric cell which in turn results in an amplified effectiveresistance or impedance to the How of AC current in leads 79 and 80through the magnetic amplifier. A resistor, indicated by the referencenumeral 82, is disposed in lead 79 between the source of electric powerand the magnetic amplifier, and a lead 84 is connected to lead 79intermediate resistor 82 and magnetic amplifier 78. It will be seen thatwith this arrangement the voltage across leads 84 and 80 varies withchanges in radiation received by photoelectric cell 74. A lead 86,connected to lead 79, together with lead 84 supplies power to anadjustable auto-transformer 88 which in turn is connected to a full waverectifier indicated by the reference numeral 89. Rectifier 89 supplies acontrolled rectified voltage to motor 52 through commutator rings, notillustrated, and leads 90 and 91. The voltage supplied to motor 52 withthis arrangement can be manually controlled by means of variabletransformer 88 and is automatically varied in response to variations inthe light energy projected upon the surface of photoelectric cell 74.

In operation, a large rod of semiconductor material is inserted in rodholder 10 and properly aligned in a vertical position. Verticalalignment can be checked by rotating rod holder 10 and noticing theamount of lateral movement at the upper end of the rod of semiconductormaterial. In most instances it is desirable to insert a piece ofmolybdenum or other electrically conductive material in rod holder 10along with the rod of semiconductor material 12 to act as a preheatersince hyperpure semiconductor materials are normally such poorconductors at low temperatures that an effective coupling cannot beaccomplished with a high frequency heater coil and a segment of thesemiconductor material must be heated to a relatively high temperatureat which it becomes conducting :before an effective couple can beestablished.

With the rod of semiconductor material 12 in proper position a seed rodof small diameter is inserted between rollers 28 and 30 and lowereduntil it is slightly above the upper end of the rod of material held byrod holder 10. The apparatus is then sealed so that the zone refiningoperation can be conducted in an inert atmosphere or in a vacuum asdesired.

To begin the zone refining operation, heater coil 62 is lowered or rodholder 10 is raised until the heater coil is even with a piece ofmolybdenum or the like held by the rod holder so that the piece ofmolybdenum is, when a high frequency current is passed through coil 62,heated to a high temperature and in turn heats the lower extremity ofrod 12 to a temperature such that it becomes electrically conductive. Inthe case of silicon this is usually a red heat. While continuing theflow of a high frequency current through coil 62 it is then movedupwardly relative to holder 10 at a very slow rate so that the coupleestablished between the heater coil and the rod of semiconductormaterial is not broken. By this means the hot area of rod 12 can bemoved from its lower extremity to its upper extremity. When the heatercoil is approximately even with the upper extremity of the rod ofsemiconductor material held by holder 10, the current being passedthrough coil 62 is increased so that the upper extremity of the rod ofsemiconductor material held by holder 10 becomes molten. At this timethe seed rod of semiconductor material held by rollers 28 and 30 islowered to contact the upper molten extremity of the rod ofsemiconductor material held by holder 10. The apparatus is now ready forautomatic operation and for the production of a small diameter rod fromthe large diameter rod inserted in holder 10.

With the seed rod held by rollers 28 and 30 is contact with the moltenupper extremity of the rod held by holder 10, motors 58, 24 and 52 areplaced in operation and the rate of operation of motors 24 and 52 arecorrelated so that the upward movement of the lower solid extremity ofrod 12 relative to the rate of upward movement of the upper solidextremity of rod 12 (it being understood that the upper solid extremityof rod 12 was initially provided by the seed rod inserted betweenrollers 28 and 30) is such that a small diameter rod of the desired meandiameter is withdrawn from the molten zone 66. As the lower extremity ofrod 12 is pushed upwardly into the molten zone by means of motor 24, theupper solid extremity of rod 12 is withdrawn from the molten zone bymotor 52 at a rate such that the two solid portions of the rod arecontinuously receding from each other, and the large diameter feed stockis transformed into a small diameter product by means of the zonerefining operation. It will thus be seen that since the lower solidextremity of rod 12 is being continually fed into molten zone 66 and theupper solid portion of rod 12 is being continually withdrawn from moltenZone 66, the molten zone is bounded at the bottom by an interface ofmelting and at the top by an interface of resolidification.

It was previously mentioned that movement of arms 34 and 38 relative toshafts 40 and 42 is preferably not friction free. It will now be seenthat the reason for this is that, in normal operation, friction betweenshaft 40 and arm 34 and between shaft 42 and arm 38 results in rollers28 and 30 being urged toward each other so that the upper solid portionof rod 12 is gripped more forcefully. This reduces the possibility ofslippage between rod 12 and rollers 28 and 30 and reduces the extent towhich rollers 28 and 30 need be spring biased toward each other.

Once a proper pull rate, i.e., the rate at which the upper solidextremity of rod 12 is withdrawn from the molten zone 66, isestablished, lens 76 is focused upon the interface of resolidification68 so that an image of a surface area of rod 12 isprojected upon thesurface of photoelectric cell 74. With experience, proper positioning ofthe lens 76 can be accomplished prior to the initiation of the zonerefining operation since with a selected diameter of feed stock andproduct, the position of the interface of resolidification can beaccurately estimated. If for any reason, such as fluctuations in thediameter of the lower section of rod 12 being fed into molten zone 66 orfluctuations in the current being passed through coil 62, there ismovement of the interface 68 relative to heater coil 62 so that there isa change in d,

the radiation being received by the surface of cell 74 changes so thatthe resistance of photocell 74 changes and the speed of pull motor 52 isincreased or decreased as the case may be. If the illumination onphotocell 74 is increased due to a downward movement of interface 68(radiation from the surface of the molten zone 66 is lower per unit areathan from the surface of adjacent solid portions of rod 12), the speedof motor 52 is increased due to a decrease in the resistance of thephotocell and an increase in voltage across leads 84 and 80, and if theillumination on photocell 74 is decreased due to a relative upwardmovement of interface 68, the rate of operation of motor 52 is decreasedto thereby reduce the pull rate. As previously mentioned, it has beenfound that if factors which result in a relative downward movement ofinterface 68 are allowed to persist, an increase in the diameter of thesegment of rod 12 being withdrawn from molten zone 66 results, but areadily discernible movement of interface 68 occurs prior to anysubstantial change in the diameter of the rod segment above the moltenzone. Thus, factors which normaly would result in an increase in roddiameter can be corrected, under favorable conditions, before anysubstantial increase occurs. In a similar manner undesirable decreasesin the diameter of the rod segment above molten zone 68 are alsocontrolled so that there is produced by solidification of the moltenzone a rod segment varying only within narrow limits from a selecteddiameter.

With particular reference to FIGURE 3 of the drawings there isillustrated an arrangement in Which two photoelectric cells are employedin series to control the rate of operation of a motor corresponding tomotor 52 in FIG- URE l of the drawings so that the pull rate isdetermined both by the position of the interface of resolidification andby the diameter of the rod of semiconductor material. With particularreference to this figure of the drawings there is illustrated a rod ofsemiconductor material 92 in which there is formed a molten zone 93which is bounded at its upper extremity by an interface ofresolidification 94 and at its lower extremity by an interface ofmelting 96. A first photoelectric cell 98, which corresponds generallyto photoelectric cell 74 in FIGURE 1 of the drawings, receives radiationfrom a surface area of rod 92 including a portion of molten zone 93 anda portion of the rod 92 above interface 94. A second photoelectric cellis positioned such that it receives, over approximately 50 percent ofits surface, radiation from rod 92 and over the remainder of its surfacereceives only background radiation. The photoelectric cells 98 and 100are electrically connected in series so that the resistance across thetwo cells decreases in response to a downward movement of interface 94and/or in response to a decrease in diameter of the rod 92. With thisarrangement the pull rate is less sensitive to movement of the interfaceof resolidification 94 but changes in diameter which result from largefluctuations in the diameter of rod 92 at the interface of melting 96are also controlled. This arrangement, therefore, is of particular valuewhen Working with feed stock of irregular diameter.

With specific reference to FIGURE 4 of the drawings there is illustratedan arrangement similar to that shown in FIGURE 3 except that only asingle photoelectric cell is employed. In this figure of the drawingsthe reference numeral 102 indicates a rod or semiconductor material inwhich a molten zone 104 is formed by a suitable heater, not illustrated,the molten zone being bounded at its upper extremity by an interface ofresolidification 106 and at its lower extremity by an interface ofmelting 108. A photoelectric cell 110 is so arranged. that it normallyreceives light over about of its surface from a surface area of rod 102selected to include adjacent portions of the solid rod above interface106 and of the molten zone below interface 106, and such that over theremaining Vs of its surface it normally receives only backgroundradiation. With this arrangement the photoelectric cell 110 receives anincreasing amount of radiation as interface 106 is lowered relative tothe photoelectric cell or as rod 102 increases in diameter so that thepull rate is dependent upon both rod diameter and the position ofinterface 106.

It will be understood that rod diameter control apparatus in accordancewith this invention can be utilized with any zone refining apparatus,whether of the gas or vacuum type. As previously mentioned, however, itis particularly advantageous for use with zone refining apparatusdesigned to produce small diameter rods having a mean diameter of lessthan about 4 or 5 millimeters. It will likewise be understood thatvarious modifications may be made in the electrical, mechanical oroptical systems of the embodiments illustrated without departing fromthe spirit of the invention.

Having thus described my invention and several specific embodimentsthereof, what I desire to claim and secure by Letters Patent is:

1. In a method of zone melting a rod of semiconductor material in whichthe rod is vertically supported at both ends and a transverse moltenzone is formed in said rod intermediate the two ends thereof, saidmolten zone being caused to move relative to said rod along thelongitudinal axis thereof so that it is bounded at one extremity by atransverse interface of melting and at the other extremity by atransverse interface of resolidification, and in which the diameter ofthe rod being solidified from said molten zone is controlled by movementrelative to each other of the solid portions of said rod on either sideof said molten zone, the improvement which comprises continuallydetecting variations in the distance between at least one point on saidinterface of resolidification and an arbitrarily selected referenceplane passing transversely through the longitudinal axis of said rod andmoving relative to said rod along said longitudinal axis at a uniformrate equal to the mean rate of movement of said molten zone relative tosaid rod, and moving the solid portions of said rod on either side ofsaid molten zone relative to each other in response to changes in saiddistance to thereby produce by solidification of said molten zone a rodsegment varying only within narrow limits from a selected diameter.

2. In a method of zone melting a rod of semiconductor material in whichthe rod is vertically supported at both ends and a transverse moltenzone is formed in said rod intermediate the two ends thereof, saidmolten zone being caused to move relative to aid rod along thelongitudinal axis thereof so that it is bounded at one extremity by atransverse interface of melting and at the other extremity by atransverse interface of resolidification, and in which the diameter ofthe rod being solidified from said molten zone is controlled by movementrelative to each other of the solid portions of said rod on either sideof said molten zone, the improvement which comprises sensing theradiation emitted from a surface area of said rod selected to include aportion or the surface of said molten zone and an adjacent portion ofthe surface of said rod resolidified from said molten zone, saidselected surface area being a constant mean distance from an arbitrarilyselected reference plane passing transversely through the longitudinalaxis of said rod and moving relative to said rod along said longitudinalaxis at a uniform rate equal to the mean rate of movement of said moltenzone relative to said rod, whereby movement of said interface ofresolidification relative to said reference plane results in acorresponding change in the amount of energy being radiated from saidselected surface area, and moving the solid portions of said rod oneither side of said molten zone relative to each other in response toradiation sensed from said selected area to thereby produce bysolidification of said molten zone a rod segment varying only withinnarrow limits from a selected diameter.

3. A method as in claim 2 wherein relative movement is effected betweenthe upper solid portion of said rod and the lower solid portion of saidrod such that during at least a selected portion of the zone refiningoperation the two solid portions of the rod are continuously recedingfrom each other, whereby the diameter of said rod at said interface ofresolidification is less than the diameter of said rod at said interfaceof melting.

4. A method according to claim 3 wherein the diameter of said rod atsaid interface of resolidification is less than about 4 millimeters.

5. A method according to claim 4 wherein the rate of movement of thelower solid portion of said rod relative to said molten zone isrelatively uniform and wherein the rate of movement of the upper solidportion of said rod relative to said molten zone is varied to maintainthe diameter of said rod at said interface of resolidificationsubstantially constant.

6. In an apparatus for zone melting a rod of semiconductor materialcomprising upper and lower holding means for supporting the ends of arod of semiconductor material such that the longitudinal axis of saidrod extends vertically, heating means for providing a transverse moltenzone in said vertically extending rod of semiconductor material,tranverse means for effecting relative movement of said heating meanslongitudinally of said vertically extending rod of semiconductormaterial so that said molten zone is bounded at one extremity by atransverse interface of melting and at the other extremity by atransverse interface of resolidification, and means for effecting atcontrolled rates relative movement between the upper and lowersolidified portions of said rod of semiconductor material to therebydetermine the diameter of said rod at said interface ofresolidification, the improvement which comprises means for sensingvariations in the distance of said transverse interface ofresolidification from a reference plane passing transversely throughsaid rod of semiconductor material in fixed positional relationship tosaid heating means, and means responsive to said sensing means tocontrol the rate of movement of said upper and lower solidified portionsrelative to each other to thereby produce by solidification of saidmolten zone a rod segment varying only within narrow limits from aselected diameter.

7. Apparatus according to claim 6 wherein said sensing means comprisesmeans for measuring the radiation emitted from a selected surface areaof said rod, said area being a fixed mean distance from said referenceplane and including a portion of the surface of said molten zone and anadjacent portion of the surface of the portion of said rod resolidifiedfrom said molten zone.

8. Apparatus according to claim 7 wherein said radiation measuring meanscomprises a photoelectric cell.

9. Apparatus according to claim 7 including means for moving the solidportion of said rod resolidified from said molten zone relative to saidheating means at a rate in excess of that at which the yet to be meltedportion of said rod and said heating means approach each other, wherebya zone melting operation conducted with said apparatus results in areduction in the diameter of a rod of semiconductor material.

10. Apparatus according to claim 9 wherein said radiation measuringmeans comprises a photoelectric cell.

11. Apparatus according to claim 10 wherein said radiation measuringmeans additionally comprises a lens system for producing an image uponsaid photoelectric cell of said selected surface area of said rod ofsemiconductor material.

12. Apparatus according to claim 11 including support means for saidheating means and wherein said lens system and said photoelectric cellare carried by said support means.

13. In an apparatus for zone melting a rod of semiconductor materialcomprising upper and lower holding means for supporting a rod ofsemiconductor material such that the longitudinal axis of said rodextends vertically, heating means for providing a transverse molten zonein said vertically extending rod of semiconductor material, traversemeans for effecting relative movement of said heating meanslongitudinally of said vertically extending rod of semiconductormaterial so that said molten zone is bounded at one extremity by atransverse interface of melting and at the other extremity by atransverse interface of resolidification, and such that said rodcomprises a solid portion resolidified from said molten zone and asecond solid portion which is yet to be melted, and means for movingsaid first named solid portion and said heating means relatively awayfrom each other at a rate in excess of that at which said second namedsolid portion and said heating means approach each other so that thediameter of said rod is reduced as a result of its being zone melted,the improvement which comprises means for measuring the radiationemitted from a selected surface area of said rod, said area being afixed mean distance from a reference plane passing transversely throughsaid rod of semiconductor material and centrally through said heatingmeans, said reference plane being stationary relative to said heatingmeans, and said selected surface area including a portion of the surfaceof said molten zone and an adjacent portion of the surface of said firstnamed solid portion of said rod, and control means responsive to saidradiation measuring means to decrease the rate of movement of said twosolid portions of said rod relative to each other when the radiationemitted from said selected area decreases below a predetermined leveland to increase the rate of movement of said solid portions of said rodrelative to each other when the radiation emitted from said selectedarea, increases above another predetermined level.

14. Apparatus according to claim 13 wherein said radiation measuringmeans comprises a photoelectric cell and a lens system for producing animage upon said photoelectric cell of said selected surface area of saidrod.

15. Apparatus according to claim 13 including diameter measuring meansfor sensing changes in the diameter of said rod at said interface ofresolidification and wherein said control means is responsive both tosaid radiation measuring means and said diameter measuring means.

16. Apparatus according to claim 15 wherein said radiation measuringmeans and said diameter measuring means in each instance comprises aphotoelectric cell and a lens system for producing an image upon saidphotoelectric cell of a selected portion of the surface of said rod.

17. Apparatus according to claim wherein said radiation measuring meansand said diameter measuring means comprise a single photoelectric celland a lens system for projecting an image upon said photoelectric cellof a portion of the surface of said rod, said portion increasing as thediameter of said rod at said interface of resolidification increases anddecreases as the diameter of said rod at said interface ofresolidlification decreases.

References Cited UNITED STATES PATENTS 2,913,561 11/1959 Rummel et al23-301 3,046,379 7/1962 Keller et al 23--301 3,136,876 6/1964 Crosthwait23301 3,157,472 11/1964 Kappelmeyer et al. 23-273 3,190,727 6/1965Vunderink 23-273 3,190,728 6/1965 Vunderink 23- 273 WILBUR L. BASCOMB,JR., Primary Examiner.

G. P. HINES, Assistant Examiner.

US. Cl. X.R.

1. IN A METHOD OF ZONE MELTING A ROD OF SEMICONDUCTOR MATERIAL IN WHICHTHE ROD IS VERTICALLY SUPPORTED AT BOTH ENDS AND A TRANSVERSE MOLTENZONE IS FORMED IN SAID ROD INTERMEDIATE THE TWO ENDS THEREOF, SAIDMOLTEN ZONE BEING CAUSED TO MOVE RELATIVE TO SAID ROD ALONG THELONGITUDINAL AXIS THEREOF SO THAT IT IS BOUNDED AT ONE EXTREMITY BY ATRANSVERSE INTERFACE OF MELTING AND AT THE OTHER EXTREMITY BY ATRANSVERSE INTERFACE OF RESOLIDIFICATION, AND IN WHICH THE DIAMETER OFTHE ROD BEING SOLIDIFIED FROM SAID MOLTEN ZONE IS CONTROLLED BY MOVEMENTRELATIVE TO EACH OTHER OF THE SOLID PORTIONS OF SAID ROD ON EITHER SIDEOF SAID MOLTEN ZONE, THE IMPROVEMENT WHICH COMPRISES CONTINUALLYDETECTING VARIATIONS IN THE DISTANCE BETWEEN AT LEAST ONE POINT ON SIADINTERFACE OF RESOLIDIFICATION AND AN ARBITRARILY SELECTED REFERENCEPLANE PASSING TRANSVERSELY THROUGH THE LONGITUDINAL AXIS OF SAID ROD ANDMOVING RELATIVE TO SAID ROD ALONG SAID LONGITUDINAL AXIS AT A UNIFORMRATE EQUAL TO THE MEAN RATE OF MOVEMENT OF SAID MOLTEN ZONE RELATIVE TOSAID ROD, AND MOVING THE SOLID PORTIONS OF SAID ROD ON EITHER SIDE OFSAID MOLTEN ZONE RELATIVE TO EACH OTHER IN RESPONSE TO CHANGES IN SAIDDISTANCE TO THEREBY PRODUCE BY SOLIDIFICATION OF SAID MOLTEN ZONE.